Method of imaging by SPECT

ABSTRACT

A method of imaging a target organ in a patient by SPECT, by using a gamma camera having a gamma detector provided with a fan-beam collimator ( 2 ), focusing to a focal line parallel to the patient&#39;s body length, and by computer reconstructing the distribution of the radioactivity inside the patient&#39;s body from the acquired planar images by using certain reconstruction algorithms. The images are acquired along at least one linear orbit performed in a direction perpendicular to the patient&#39;s body, and the collimator focal line is made to travel throughout said target organ during the acquisition. The invention further relates to an equipment for performing this method.

METHODS OF IMAGING BY SPECT

The invention relates to a method of imaging a target organ in a patientby SPECT, by using a ‘gamma camera having a gamma detector provided witha fan-beam collimator, focusing to a focal line parallel to thepatient's body length, and by computer reconstructing the distributionof the radioactivity inside the patient's body from the acquired planarimages by using certain reconstruction algorithms.

The Single Photo Emission Computed Tomography (SPECT) is routinely usedin clinical studies. SPECT is performed by using a gamma camera,comprising a collimator fixed on a gamma detector, which gamma camerafollows a revolution orbit around the patient's body. The gamma rays,emitted by a radioactive tracer, accumulated in certain tissues ororgans of the patient's body, are sorted by the collimator and recordedby the gamma detector under various angles around the body, thecollimator always pointing to (facing) the rotation axis, of the camera.From the acquired planar images the distribution of the activity insidethe patient's body can be computed using certain reconstructionalgorithms. Generally the so-called Expectation-Maximization of theMaximum-Likelihood (EM-ML) algorithm is used, as described by Shepp etal. (IEEE Trans. Med. Imaging 1982; 2:113-122) and by Lange et al. (J.Comput. Assist. Tomogr. 1984; 8:306-316). This iterative algorithmminimizes the effect of noise in SPECT images.

The collimators nowadays is use are manufactured from a lead sheatperforated with a plurality of usually parallel holes. The collimator isthe most problematic element of the SPECT device, with regard to itspoor sensitivity (less than 0.01% of the gamma radiation passes thecollimator and reaches the detector) and its poor spatial resolution,becoming increasingly worse with increasing distance between activitysource (i.e. the organ or tissue wherein the radioactivity has beenaccumulated) and collimator. Improvement of one of these properties,e.g. by modifying the hole length or diameter of the collimator, isalways to the detriment of the other one. Furthermore, the SPECTtechnique is inadequate in producing reliable images because of the factthat small fluctuations in the acquired data can involve significantvariations in the reconstructed images. This is due to the geometry ofthe acquired data. The limited time available for obtaining thenecessary information (because of the restricted fixation time of thepatient and the decay time of the radioactive tracer) and the limitedinjected radioactivity dose (limited for health care reasons) lead toacquired images containing statistical noise. Indeed the measurement ofa radioactive process follows the Poisson law, giving a signal to noiseratio proportional to the square root of the count rate. As a result,the reconstructed images are frequently corrupted by significant falsepositive information, so-called noise artefacts. Consequently, it is amajor goal in SPECT imaging to increase the SPECT sensitivity withoutreduction of the spatial resolution in order to improve the acquiredsignal to noise ratio.

In an attempt to improve the sensitivity-resolution couple of thecollimator, fan-beam collimators, focusing to a focal line, have beendeveloped recently: see e.g. the review articles by Moore et al. (Eur.J. Nucl. Med. 1992; 19:138-150) and by Rosenthal et al. (J. Nucl. Med.1995; 36:1489-1513). These collimators, having holes converging in onedimension to a focal line, have an increasingly bettersensitivity-resolution couple when the activity source approaches thecollimator focal line. By using a fan-beam collimator in the SPECTimaging technique, acquiring the images along the classical revolutionorbit, the focal line is parallel to the axis of rotation of the gammacamera on the other side of the patient and consequently parallel to thepatient's body length (see the above publication by Rosenthal et al., p.1495). Nevertheless, the activity source, i.e. the target organ, hasonly a restricted approaching range with regard to the collimator focalline, because said organ and all activity contained in the sametransverse (i.e. perpendicular to the patient's length) slice must bekept within the collimator acceptance angle during the acquisition bythe rotating camera. Otherwise, the reconstructed images are corruptedby significant truncature artefacts. This problem of image truncation byusing fan-beam collimators is discussed in more detail by Manglos et al.(Phys. Med. Biol. 1993; 38:1443-1457) and by Kadrmas et al. (Phys.Me.Biol. 1995; 40:1085-1101). The above requirement, viz. to keep allsource activity, i.e. in fact the complete body diameter of the patient,within the collimator acceptance angle during the acquisition along arevolution orbit, limits the choice of fan-beam collimators to thosehaving a relatively large focal length, viz. greater than approx. 60 cm,giving results not very different from those obtained with a parallelcollimator. Therefore, the target organ cannot be positioned close tothe focal line of the collimator where its sensitivity and spatialresolution are optimal. As a consequence, the sensitivity improvement,obtained by this technique for similar resolution, is limited to afactor of approx. 1.5 at most. Also the target of interest must besmaller than the detector transverse slice (preferably approx. 1.4 timessmaller).

It is the objective of the present invention to provide a method ofimaging by SPECT with a substantially improved sensitivity-resolutioncouple. In other words, it is the aim of the present invention toprovide a method of SPECT imaging which results in substantiallyimproved reconstructed images.

This objective can be achieved by a method as defined in the openingparagraph, viz. a method of SPECT imaging a target organ in a patient,by using a gamma camera having a gamma detector provided with a fan-beamcollimator, focusing to a focal line parallel to the patient's body,followed by computer reconstruction of the radioactivity distributionfrom the acquired images, which method according to the presentinvention is characterized in that the images are acquired along atleast on linear orbit performed in a direction perpendicular to thepatient's body length, and in that the collimator focal line is made totravel throughout said target organ during the acquisition.

It has surprisingly been found, that by applying the above method,wherein the usable transverse size dimension of the SPECT device can nowbe fully used (i.e. the target organ size has now only to be equal atmost to the detector transverse size, because the target organ has nolonger to be kept within the collimator acceptance angle during theacquisition) the acquired set of planar images is complete (i.e.sufficient to reconstruct the activity distribution) and thatconsiderable improvements with regard to the sensitivity-resolutioncouple can be obtained. The advantages will be evident. Betterreconstructed images can be obtained by using the same acquisition timeand the same dose of injected radioactivity. In this manner lesions orother malignancies in the body of a patient can be detected earlier, forexample, metastasation of tumours in an early stage of development. Atchoice, however, the acquisition time can be reduced considerably toobtain, with the same dose of injected radioactivity, images suitablefor routine investigations. This results in a reduction of the costs forthe clinic or hospital. Also at choice, as a third alternative the doseof injected radioactivity can be reduced in order to burden the patientto a lesser extent. Optionally these advantages can be reached incombination with each other, then, or course, to a somewhat lesserextent but nevertheless with sufficiently attractive prospects.

Preferably, to reach superior results, the images are acquired by themethod of the present invention along four linear orbits which areperformed in mutually transverse directions perpendicular to thepatient's body.

It should be emphasized, that by the term “target organ” is meant theorgan or tissue to be studied or investigated by using the method of theinvention. The term “target organ” obviously encompasses a plurality oforgans to be studied simultaneously and also a part of the body, likethe head, the chest or the abdomen, or even the complete body of thepatient.

It is further important to note, that the linear orbits must notnecessarily be straight lines, but also encompass slightly curved lines.The expression “at least substantially straight lines” may besatisfactory in this connection. The variations of the linear orbitswith respect to straight lines, however, must be small to meet therequirement, that the collimator focal line is made to travel throughoutthe target organ during the acquisition.

It has been observed, that the quality of the reconstructed images canfurther be improved, if during the acquisition the fan-beam collimatorremains parallel to its initial position along each orbit. This caneasily be reached by shifting the collimator during the acquisitionaccurately parallel to the patient's body, or vice versa.

The method according to the present invention is not restricted to theuse of one gamma detector provided with a fan-beam collimator(detector-collimator combination, detector-fixed fan-beam collimator),but encompasses the use of up to four detector-collimator combinations,in particular of two and four combinations additionally. More gammacameras can be used in that case or, if desired, a two-headed orfour-headed camera, i.e. a camera with two or four detector-collimatorcombinations. Of course, all collimators should be of the fan-beam type,focusing to a focal line. If a second detector-collimator combination isapplied, this combination is used, simultaneously with and positionedopposite to the first one, sandwiching the patient in between.

If the use of four detector-collimator combinations is preferred, twocouples of mutually opposite gamma detectors provided with fan-beamcollimators are used simultaneously and in mutually perpendicularposition, both couples sandwiching the patient in between; the imagesare acquired by moving each of the detector-collimator combinationsalong a linear orbit.

It has been observed, that by using a plurality of detector-collimatorcombinations, in particular two or four, according to the presentinvention, simultaneously following the various linear orbits, thesensitivity of the SPECT device can further be improved, resulting instill better reconstructed images.

Due to the fact that in the method of the invention the collimator focalline is made to travel throughout the target organ, so remains withinthe patient's body during acquisition, a fan-beam collimator or aplurality of fan-beam collimators can be used with a considerablyreduced focal length, more in particular a focal length of betweenapprox. 12 and approx. 30 cm, preferably of approx. 25 cm. As a result,the patient to be examined and also the target organ or organs can noweasily be positioned within the collimator focal line where both thesensitivity and the resolution are optimal. In this pre-eminentlysuitable method of the invention, wherein a considerably reducedcollimator focal length is used, the sensitivity can in principle beimproved with a factor of approximately 10 compared with the best actualsystem, if a same spatial resolution is applied. This sensitivity evenfurther increases when the size of the studied organ decreases. Inaddition, the reduction of the usable transverse slice size, needed toavoid image truncation, as observed in the usual SPECT technique usingfan-beam collimators, is no longer present.

To improve their results, gamma cameras for SPECT imaging are oftenadapted to the special organs to be studied (organ-dedicated), forexample, head-dedicated equipment for specific study of the head (byusing an annular camera), etc. If in the method of the inventionhead-dedicated cameras are preferred, such cameras have only to be beequipped with fan-beam collimators with a focal length of approx. 12 cm.The method of the present invention, however, gives so much betterreconstructed images, that this method is well applicable for the wholebody of a patient as well as for only a part of the body, e.g. the head,without adverse effects on the quality of these images. Therefore, themethod of the invention can be considered as universally applicable orallround, in that fan-beam collimators with a focal length ofapproximately 25 cm can be used generally, i.e. both for the whole bodyand for organ-dedicated SPECT imaging.

In a favourable embodiment, the method of the present invention isperformed by using at least one fan-beam collimator as disclosed in U.S.Pat. No. 5,198,680 (Kurakake et al.) Such a fan-beam collimatorcomprises first septa members arranged in a fan-shape pattern in whichall the first septa members are oriented towards a common focal line;and second septa members arranged to be parallel to each other, whichsecond septa members are perpendicularly crossing with the first septamembers in a lattice shape such that holes with a longitudinal crosssection are defined between each adjacent first septa members and eachadjacent second septa members.

It has further been found, that the distribution A(x,y,z) of theradioactivity in the patient's body can be computed using the followingnew reconstruction algorithm (this is in fact the mathematical proofthat the acquired set of planar images is complete, i.e. sufficient toreconstruct the distribution activity): $\begin{matrix}{{A\left( {x,y,z} \right)} = {\frac{1}{\left( {2\pi} \right)^{2}f}{\int_{- f}^{f}{{r}\quad \left( {{{\overset{\sim}{P}}_{left}\left( {{{\frac{r}{f}x} + y},r,z} \right)} + {{\overset{\sim}{P}}_{under}\left( {{{\frac{r}{f}y} + x},r,z} \right)} + {{\overset{\sim}{P}}_{right}\left( \quad {{{\frac{r}{f}x} + y},r,z} \right)} + {{\overset{\sim}{P}}_{over}\left( {{{\frac{r}{f}y} + x},r,z} \right)}} \right)}}}} & (I) \\{{{\overset{\sim}{P}}_{\alpha}\left( {W,r,z} \right)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{k}\quad ^{ikW}{k}^{- {ikU}_{\alpha \frac{r}{f}}}{\int_{- \infty}^{+ \infty}{{V}\quad ^{- {ikV}}{P_{\alpha}\left( {V,r,z} \right)}}}}}}} & \quad\end{matrix}$

wherein:

x, y and z are the orthogonal coordinates along the horizontaltransverse direction, the vertical transverse direction and thelongitudinal direction, respectively;

P_(α)(V,r,z) are the planar images pixels values, where r is thecoordinate along the transverse direction of the detector and V is thedetector position along the linear orbit a;

f is the fan-beam collimator focal length; and

U_(α) is the shift length of the fan-beam collimator focal line in thelinear orbit a versus the origin of the axis coordinates (x=0, y=0), thesaid origin being located for x and y respectively at the middle of thetwo collimator positions in orbits left and right (under and over theradiation source, respectively).

The possibility of using a different shift length U_(α)for each linearorbit a allows to choose a special patient body region of interest,through which the collimator focal line travels during the said linearorbit a acquisition. This region can be the same for the four linearorbits in order to have the optimal sensitivity-resolution couple inthis region. Alternatively, the collimator focal line can travel througha different region of interest in each linear orbit to share a highsensitivity-resolution couple among a more extended region. Furthermore,each linear orbit can be repeated with various shift lengths U_(α),reconstructed by using the algorithm disclosed hereinbefore, and thensummarised to further extend the region which shares the maximumsensitivity-resolution couple.

The limit [−f,f] in the integration dr shows that the transverse size ofthe detector must be greater than two times the collimator focal length,according to the above algorithm. It is also important to point out thatP_(α)(V,r,z) vanishes when the target organ does no longer intersept thecollimator acceptance angle, and thus the integration dV, and as aresult also the acquisition orbit range can be reduced, allowing anincreasing acquisition time per planar image, i.e. an increasingsensitivity, for a same total acquisition time.

The above algorithm is the exact reconstruction of the acquired imagesunder the assumption that the collimator resolution, the gammaattenuation and the gamma scatter can be neglected. If these effectsshould be taken in account, certain well-known iterative algorithms,like EM-ML (see hereinbefore) can additionally be used forreconstruction purposes.

In case of radioactive sources inside a homogeneous attenuation medium,the so-called Bellini method (IEEE Trans Signal Proc. 1979; 27(3):213-218) is applicable, and leads to a projection free of attenuationP^(o)(α), using the following substitution in the fourier space of theabove formula I: $\begin{matrix}{{P_{\alpha}^{o}\left( {k,r,z} \right)} = {P_{\alpha}\left( {k,\frac{r - {i\frac{\mu}{k^{2}}\sqrt{r^{2} + {f^{2}\left( {1 + \frac{\mu^{2}}{k^{2}}} \right)}}}}{1 + \frac{\mu^{2}}{k^{2}}},z} \right)}} & ({II})\end{matrix}$

wherein μ is the attenuation coefficient.

The invention also relates to an equipment for performing the abovemethod of SPECT imaging according to the invention, comprising at leastone gamma camera with at least one detector-fixed fan-beam collimator,and a bed for a patient to be examined in such a relative position, thatthe bed is surrounded by four collimator positions, essentially situatedat the angular points of a square (which are only for simplicity reasonschosen to be situated over the bed (a), under the bed (b), and on bothsides (c) and (d) of the bed), which positions can be occupied by saidat least one collimator focusing to a focal line parallel to the bedlength. The usual equipment for imaging a patient by SPECT comprises agamma camera with one or two (two-headed) detector-fixed collimators,which follow a revolution orbit around the patient's body. The patientto be examined is fixedly positioned on a bed. During the revolution thecollimator continuously points to (faces) the body of the patient andoccupies successively all collimator positions of the revolution orbit,so including the above-defined four collimator positions. If a fan-beamcollimator is used in this traditional revolution orbit technique, saidcollimator focuses in each of these positions to a focal line parallelto the axis of rotation of the gamma camera on the other side of thepatient and consequently parallel to the patient's body (seehereinbefore).

According to the present invention, the equipment for performing theabove method of imaging by SPECT is characterized in that:

the bed is positionable at such a distance from the collimatorpositions, that in each position the collimator focal line is inside thepatient's body on the bed; and

the bed is adapted to allow movements vis-à-vis said at least onecollimator in two perpendicular directions, both transverse to the bedlength, viz. a sideward movement at position a or b of said at least onecollimeter and an up and downward movement at position c or d thereof;or, alternatively, said at least one collimator is adapted to allowmovements vis-à-vis the bed in perpendicular directions, all transverseto the bed length, viz. substantially parallel to the bed surface in thepositions a and b, and substantially perpendicular to the bed surface inthe positions c and d.

By positioning the bed at such a distance from the fan-beam collimatorpositions (this positioning can be adjusted by a computer, preferably bythe acquisition computer), that in each of these positions thecollimator focal line is inside the patient's body on the bed, thecollimator focal line travels through the patient's body or the targetorgan therein during the acquisition by the gamma camera along linearorbits. By adapting the bed or the fan-beam collimator in such mannerthat it allows relative perpendicularly directed movements, as describedabove, images can be acquired by the gamma camera along four linearorbits performed in mutually transverse directions perpendicular to thepatient's body.

The range of the relative movements of the bed vis-à-vis the collimatoror collimators should preferably be at least equal to two times thetransverse size of the detector or collimator, and should preferablyamount to approximately 100 cm. As is already explained hereinbefore,the fan-beam collimator(s) forming part of the equipment of theinvention has (have) advantageously a focal length of between approx. 12and approx. 30 cm. If allround, i.e. not dedicated to the imaging ofcertain target organs or parts of the body like the head, the focallength is preferably approx. 25 cm.

It should be emphasized that by the expression “at least one” should beunderstood: one up to four; more in particular: one, two or four.

So the equipment according to the present invention may convenientlycomprise one gamma detector provided with a fan-beam collimator. Such adetector-collimator combination is equipped in such manner that it canbe moved from the above-defined position a to positions c, b and d,successively, and vice versa.

It may be of advantage, however, to include a second gamma detectorprovided with a fan-beam collimator into the equipment of the presentinvention. In that case the two detector-collimator combinations arepositioned opposite to each other, sandwiching bed plus patient inbetween both equipped in such manner that they can be moved fromposition a to position c, and from position b to position d,respectively, and vice versa.

In case one or two detector-collimator combinations are present in theequipment of the invention, the equipment is preferably so adapted thatthe bed is movable vis-à-vis the collimator by means of a system ofmotive members, preferably a combination of a horizontally shiftingmobile member at the foot of the bed and a jack for moving the bed intoa vertical direction. This system of motive members is explained in moredetail hereinafter.

In an equally advantageous embodiment the equipment of the presentinvention comprises four gamma detectors with fan-beam collimators,which detector-collimator combinations are so positioned that theyoccupy positions a, b, c and d, respectively, thereby sandwiching bedplus patient in between.

In this embodiment the four detector-collimator combinations arepreferably movable vis-à-vis the bed by means of a motive system,preferably a rigid frame of four mutually perpendicular rails,positioned transversally to the bed length, along which thedetector-collimator combinations can slide. This motive system is alsoexplained in the Examples.

It is another merit of the present invention that the relative movementsof the bed vis-à-vis the detector-collimator combination(s) are computercontrolled (cybernation) by the gamma camera. This advanced system ofcomputer-driven detector-collimator combination(s) relative to thepatient's bed, in which the above-defined new algorithm is convenientlyused, enables the user of the system, i.e. the personnel of the clinicor hospital, to investigate the patient full-automatically by theimproved SPECT imaging technique of the invention.

The invention is described hereinafter with reference to joint Figures,and to the detailed description of the drawings and of modelexperiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying drawings, wherein:

FIGS. 1 and 2 are schematic representations of the equipment accordingto the present invention in a suitable embodiment; FIG. 1 viewed in thelongitudinal direction of the bed and FIG. 2 viewed in a directiontransverse to the bed;

FIG. 3 is also a schematic representation of the equipment of thepresent invention, now in another suitable embodiment, viewed in thelongitudinal direction of the bed, as in FIG. 1; and

FIGS. 4 and 5 show SPECT spatial revolution images, obtained byperforming model experiments.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show a gamma detector 1 equipped with a fan-beamcollimator 2 and movably attached to a circular rail 3 held by twopylons 9. The detector 1 can move along the rail, the collimator 2always pointing to the rotation axis 8. Using a magnetic brake, thedetector 1 can be positioned over, under, left and right the bed 4:positions a, b, c and d, respectively (the collimator centres aresituated at the angular points of a sqare). A motor attached to thedetector 1 and drawing an endless screw acting on a circular rackattached along the rail 3 can be used to move the detector-collimatorcombination from one position into another. The bed 4 can verticallymove thanks to the jacks 5, which can be constituted by a motorizedendless screw acting on a rack. A crenelated plate drawing by theendless screw and inserted in an optical switch can be used to adjustthe vertical position of the bed 4. This bed can also move along theleft-right direction of FIG. 1 (horizontal transverse direction) thanksto the mobile element 7 which can be a trolley rolling along a rail onthe floor. Again a motorized endless screw acting on a rack and drawinga crenelated plate inserted in an optical switch can be used to move andadjust the transverse horizontal bed 4 position. The vertical andhorizontal positioning range of the bed 4 vis-à-vis the rotation axis 8is at least equal to two times the transverse size 6 of the detector 1.The collimator 2 focal line is parallel to the bed length and goesessentially throughout the rotation axis 8. The transverse size 6 of thedetector 1 is at least equal to two times the collimator focal length.The planar images are digitally acquired along four linear orbits: thebed 4 is moved into the various successive vertical positions, when thedetector 1 is unmoved left or right the bed 4 (in positions c or d,respectively); the bed is moved into the various successive transversehorizontal positions, when the detector 1 is unmoved over or under thebed 4 (in positions a or b, respectively). During acquisition, thedigital planar images and the vertical and horizontal digital bed 4positions are sent to the treatment computer. The distribution of theradioactivity over the patient's body A(x,y,z), wherein x,y and z arethe orthogonal coordinates along the horizontal transverse direction,the vertical direction and the longitudinal direction, respectively, canbe computed using the new reconstruction algorithm as disclosedhereinbefore.

A second detector—fan-beam collimator combination may be present inposition b of the above equipment, movable along the rail 3 fromposition b to position d and vice versa, whereas the first combinationis then movable from position a to position c and vice versa.

The embodiment shown in FIG. 3 comprises four gamma detectors 11 a, 11b, 11 c and 11 d, provided with fan-beam collimators 12 a, 12 b, 12 cand 12 d, situated over, under, left and right the bed 14 (positions a,b, c and d, respectively). Each detector can be moved along a rail (13a, 13 b, 13 c and 13 d), perpendicular to the bed 14 length; the railsare attached to each other to constitute a rigid frame.

During the acquisition the detector-collimator combinations move alongtheir rails, the bed being unmoved.

Description of Model Experiments

To acquire real acquisition data, model experiments have been carriedout. In such experiments the following requirements as to the equipmentshould be met:

(a) camera plus suitable fan-beam collimator;

(b) suitable radiation source; and

(c) camera plus collimator should be movable vis-à-vis the radiationsource or vice versa.

Ad (a). A suitable fan-beam collimator, meeting the requirements of thepresent invention, in particular a collimator having a suitable focallength, is not commercially available. Therefore one has resorted to theuse of a home-made collimator. This fan-beam collimator, having a holelength of 25 mm and a hole diameter of 1.5 mm, is deficient in variousrespects, viz.

(i) the shaped holes are not correctly dimensioned, giving a focal bandinstead of a focal line at the desired focal distance;

(ii) the number of holes is insufficient, leading to an insufficientmeasured radioactivity; and

(iii) the focal length increases as the holes are situated at a greaterdistance from the centre of the collimator.

These defects will have an unfavourable influence on the resultsobtained, in particular on the spatial resolution and/or thesensitivity.

Ad (b). As the radiation source is used a so-called Jaszczak's de luxephantom, well-known in the art of performing radioactive experiments.

Ad (c). The radiation source is movable relative to the collimator insuch manner that it enables the acquisition of images along linearorbits performed in two directions x and y (horizontal and vertical),perpendicular to the SPECT camera rotation axis z.

In the above arrangement, the method of the present invention isperformed with the radiation source centre situated at a distance ofapprox. 20 cm from the fan-beam collimator. After an acquisition time of90 minutes, the SPECT spatial resolution of FIG. 4A is obtained; thetotal number of counts is measured and amounts to 52 Mc.

In comparison, two commercially available parallel-hole collimators,viz. a low energy high resolution collimator (LEHR; hole length 40 mm,hole diameter 1.8 mm) and a low energy ultra high resolution collimator(LEUHR; hole length 45 mm, hole diameter 1.8 mm) are used in the priorart SPECT method, viz. with a gamma camera following a revolution orbitaround the radiation source. After an acquisition time of 90 minutes,the SPECT spatial resolutions are shown in FIGS. 4B and 4C,respectively; the measured numbers of counts are 27 Mc and 22 Mc,respectively.

From the Figures it can be concluded, that the spatial resolutionobtained according to the method of the invention is considerably betterthan by using the LEHR collimator and also still better than with theLEUHR one. In comparison with the LEUHR collimator, the sensitivityimprovement obtained is 2.36 (55/22) with simultaneously a significantimprovement of the spatial resolution (approx. 1.5). Such an improvementis beyond expectation in view of the deficiency of the home-madefan-beam collimator used, as explained above.

In the same manner acquisition data are obtained by using a thyroidphantom as the radiation source. By using in the method of the presentinvention again the above home-made fan-beam collimator, the SPECTspatial resolution of FIG. 5A is obtained after an acquisition time of90 minutes. By using in the prior art SPECT method the above-describedparallel-hole LEHR collimator, an approximately equal spatial resolutionis obtained after the same acquisition time: FIG. 5B. By using thecommercial parallel-hole collimator, a total number of counts of 3.1 Mcis measured, whereas, by using a collimator according to the method ofthe invention, on the other hand, a total number of counts of 16.1 Mc ismonitored, i.e. a sensitivity improvement of approximately 5.

What is claimed is:
 1. A method of imaging a target organ in a patientby SPECT, by using a gamma camera having a detector/collimatorcombination including a gamma detector provided with a fan-beamcollimator, comprising: focusing the collimator to a focal line parallelto the patient's body length; acquiring planar images by relativemovement between the gamma camera and the patient's body along at leaston linear path in a direction perpendicular to the patient's body lengthso that the collimator focal line traverses said target organ; andcomputer reconstructing the distribution of the radioactivity inside thepatient's body from the acquired planar images by using reconstructionalgorithms.
 2. The method of claim 1, wherein the images are acquiredalong four, mutually orthogonal, linear paths in directionsperpendicular to the patient's body length.
 3. The method of claim 1,wherein the collimator remains parallel to its initial position alongeach linear path.
 4. The method of claim 1, wherein twodetector/collimator combinations are used simultaneously and inpositions on opposite sides of the patient's body.
 5. The method ofclaim 1, wherein four detector/collimator combinations are usedsimultaneously and each two of the four detector/collimator combinationsare positioned on opposite sides of the patient's body, and wherein theimages are acquired by moving each of the detector/collimatorcombinations along a linear path.
 6. The method of claim 1, wherein atleast one gamma detector is used with at least one fan-beam collimatorhaving a focal length of between about 12 and 30 cm.
 7. The method ofclaim 6, wherein the focal length of the at least one fan-beamcollimator is about 25 cm.
 8. The method of claim 6, wherein thefan-beam collimator comprises first septa members arranged in afan-shape pattern in which all the first septa members are orientedtowards a common focal line, and second septa members arranged to beparallel to each other, the second septa members perpendicularlycrossing with the first septa members in a lattice shape such that holeswith a longitudinal cross section are defined between each adjacentfirst septa member and each adjacent second septa member.
 9. A method ofany one of claims 2-8, wherein the following reconstruction algorithm,is used:${A\left( {x,y,z} \right)} = {\frac{1}{\left( {2\pi} \right)^{2}f}{\int_{- f}^{f}{{r}\quad \left( {{{\overset{\sim}{P}}_{left}\left( {{{\frac{r}{f}x} + y},r,z} \right)} + {{\overset{\sim}{P}}_{under}\left( {{{\frac{r}{f}y} + x},r,z} \right)} + {{\overset{\sim}{P}}_{right}\quad\left( \quad {{{\frac{r}{f}x} + y},r,z} \right)} + {{\overset{\sim}{P}}_{over}\left( {{{\frac{r}{f}y} + x},r,z} \right)}} \right)}}}$${{\overset{\sim}{P}}_{\alpha}\left( {W,r,z} \right)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{k}\quad ^{ikW}{k}^{- {ikU}_{\alpha \frac{r}{f}}}{\int_{- \infty}^{+ \infty}{{V}\quad ^{- {ikV}}P_{\alpha}\left( {V,r,z} \right)}}}}}$

where x, y and z are the orthogonal coordinates along the horizontaltransverse direction, the vertical transverse direction and thelongitudinal direction, respectively; P_(α)(V, r, z) are the planarimages pixels values, where r is the coordinate along the transversedirection of the detector and V is the detector position along thelinear orbit a; f is the fan-beam collimator focal length; and U_(α)isthe shift length of the fan-beam collimator focal line in the linearorbit a versus the origin of the axis coordinates (x=0, y=0). 10.Apparatus for imaging a target organ in a patient by SPECT, comprising:at least one gamma camera having at least one detector/collimatorcombination including a gamma detector and a fan-beam collimator fixedto the gamma detector, a bed for a patient to be examined and having abed length; means for defining collimator positions located respectivelyat the corners of a rectangle normal to the bed length to surround thebed, and including a first position over the bed, a second positionunder the bed, a third position on one side of the bed, and a fourthposition on the side of the bed opposite the third position, the atleast one collimator being positionable at all of the first, second,third, and fourth positions, and focusing to a focal line parallel tothe bed length; means for positioning the bed at such a distance fromthe collimator positions, that the collimator focal line is inside thepatient's body on the bed in each of the collimator positions; and meansfor moving the bed in two perpendicular directions, both transverse tothe bed length, to effect a horizontal movement at the first and secondpositions of said at least one collimator and a vertical movement at thethird and fourth positions.
 11. The apparatus of claim 10, wherein themeans for positioning the bed comprises a system of motive membersincluding a horizontally shifting mobile member at the foot of the bedand a jack for moving the bed in a vertical direction.
 12. Apparatus forimaging a target organ in a patient by SPECT, comprising: at least onegamma camera having at least one detector/collimator combinationincluding a gamma detector and a fan-beam collimator fixed to the gammadetector, a bed for a patient to be examined and having a bed surfaceand a bed length; collimator positions located respectively at thecorners of a rectangle normal to the bed length to surround the bed, andincluding a first position over the bed, a second position under thebed, a third position on one side of the bed, and a fourth position onthe side of the bed opposite the third position; the at least onecollimator being positionable at all of the first, second, third, andfourth positions, and focusing to a focal line parallel to the bedlength; means for positioning the bed at such a distance from thecollimator positions, that the collimator focal line is inside thepatient's body on the bed in each of the collimator positions; and meansfor moving the at least one collimator in two perpendicular directions,both transverse to the bed length, to effect movement parallel to thebed surface at the first or second positions of said at least onecollimator and movement perpendicular to the bed surface at the thirdand fourth positions.
 13. The apparatus of any one of claims 10 or 12,wherein the range of the relative movement between the bed and the atleast one collimator is at least equal to two times the transverse sizeof said detector or collimator.
 14. The apparatus of claim 13, whereinthe range of the relative movement between the bed and the at least onecollimator is approximately 100 cm.
 15. The apparatus of any one ofclaims 10 or 12, wherein said at least one collimator has a focal lengthof between about 12 and 30 cm.
 16. The apparatus of claim 15, whereinsaid at least one collimator has a focal length of about 25 cm.
 17. Theapparatus of any one of claims 10 or 12, comprising a single gammadetector/collimator combination movable from the first position to thesecond, third and fourth positions, successively.
 18. The apparatus ofany one of claims 10 or 12 comprising two gamma detector/collimatorcombinations, positioned on opposite sides of the bed, and moveablerespectively from the first position to the third position, and from thesecond position to the fourth position.
 19. The apparatus of any one ofclaims 10 or 12, comprising four gamma detector/collimator combinationspositioned at the first, second, third and fourth positions,respectively.
 20. The apparatus of claim 19, wherein thedetector/collimator combinations are movable relative to the bed bymeans of a motive system including a rigid frame of four mutuallyperpendicular rails, positioned transversally to the bed length, alongwhich the detector/collimator combinations can slide.
 21. The apparatusof any one of claims 10 or 12, wherein the relative movement between thebed and the detector/collimator combinations are computer controlled bythe gamma camera.